The present invention relates to time delay fuses for use in electrical power distribution circuits. The fuse of the present invention is both dependable and inexpensive to construct as a result of the unique fuse construction.
Time delay fuses are often constructed such that when opening under an overload condition, a stored energy arrangement causes a portion of the fuse element to move out of the current path. This movement from the current path may result from either a spring mechanism biasing the fuse element portion or gravitational forces. In either case, isolation of the particular fuse element is required such that the commonly used arc extinguishing filler material does not contact or surround the stored energy portion of the fuse link. Examples of this type of time delay fuse are illustrated by U.S. Pat. Nos. 2,159,423; 2,577,531; 2,644,872; 2,688,676 and 3,418,614. Of these patents, only the first two rely upon a spring means for moving the fuse element out of the current path. The balance of the patents rely upon gravitational force on the heat absorbing mass opening the circuit. The common requirement throughout these patents is the necessity to isolate the heat absorbing mass from the portion of the fuse containing the arc extinguishing filler material. Clearly, the effort necessary for isolation results in added effort and expense in construction of the fuse.
The inclusion of an arc extinguishing material within the fuse enclosure is required due to the current rating of the particular type of time delay fuse being considered in each instance. Without such an arc extinguishing material, the opening of the fuse element within the enclosure at the expected current ranges could cause destruction of portions of the fuse encasement with resultant damage to the power distribution equipment. As noted above, it has been found to be desirable to isolate the low melting point mass for long time overcurrent protection from the balance of the fuse enclosure such that the arc extinguishing material is not in contact with the low melting point mass. U.S. Pat. No. 2,018,556 illustrates one attempt at constructing a fuse which does not require isolation of the low melting point mass from the arc extinguishing material. As explicitly stated in this reference, the fuse may be damaged and thus rendered inoperable if the low melting point material is raised to a temperature high enough to run without breaking the circuit. This reference found it necessary to provide a clearing agent of boric acid to thus accelerate open circuiting of the fuse in the event of a sufficient rise in temperature in the low melting point material.
U.S. Pat. No. 2,800,554 discloses a fuse having multiple link elements within a cartridge filled with arc extinguishing material. The purpose of the fuse in this instance is not necessarily to obtain time delay, but rather, to have the low melting point material located within one fuse link accelerate opening of the balance of the fuse elements due to the current through each increasing when the low melting point fuse element opens.